1. Field of the Invention
The invention relates to the field of semiconductor lasers and in particular to semiconductor lasers with nanocavities, or the use of a two-dimensional photonic crystal to localize light to a single or a selected number of defects, thus forming a high Q microcavity laser with a small modal volume.
2. Description of the Prior Art
High reflectivity mirrors have been a key ingredient to reducing the modal volume of optical laser cavities. In 1991, the operation of ultra-small cavities with lateral dimensions of 400 nm diameters was realized. The mode volume in these lasers was still rather large, approximately two cubic wavelengths due to the deep penetration of light into the mirrors.
Microdisk lasers with similar mode volumes emerged in 1993 and relied on high cavity Qs resulting from total internal reflection of light from the perimeter of the disk. In these devices, the light is guided in a thin slab, and reflects as whispering gallery modes along the circumference of the circular laser cavity. Consequently, bend losses become prohibitively large in devices with diameters below 1.5 microns.
It has been known for many years that an emitter""s spontaneous emission rate is controlled by its local electromagnetic environment, but the experimental demonstrations of controlled spontaneous lifetime at optical wavelengths are comparatively recent. This is especially true for semiconductor microcavity light emitters. Recently the benefits of using the discrete energy levels of quantum dots (QDs) to obtain microcavity controlled emission was demonstrated in a planar microcavity and then with a dielectric aperture in a semiconductor microcavity.
What is needed is a design and fabrication technique to realize these optically pumped nanocavities in room temperature, electrically pumped devices.
According to the invention it is now possible to microfabricate Bragg reflectors in one, two and even three dimensions. Two and three-dimensional microfabricated mirrors, generally referred to as photonic bandgap (PBG) crystals, can now be used to confine light to even smaller cavity volumes. In turn, this allows the definition of ultra-small subwavelength optical cavities. In such xe2x80x9cnanocavitiesxe2x80x9d, the mode volume can be as small as 2.5 cubic half wavelengths or 0.3 xcexcm3 and spontaneous emission can be efficiently coupled into the lasing mode. This efficient coupling in turn results in advantages in low noise, high modulation speed and very low threshold powers. These low threshold sources are very versatile and fast sources which can in turn be coupled together or to other nano-optic devices.
The defect cavity is illustrated here utilizing a half wavelength thick high-index membrane to confine light vertically by way of total internal reflection similar to the design of a whispering gallery microdisk laser. The high index slab is then perforated with an hexagonal array of air holes, which Bragg reflects the light in plane. A defect is formed in the two-dimensional photonic lattice by removing an air hole and/or adjusting the diameters of the few neighboring air holes. A mode or set of modes depending on the defect geometry, which is highly localized to the defect region, is formed. Photons can escape from the defect cavity by tunneling through the two dimensional photonic crystal, or by leaking out vertically from the waveguide.
It is now possible, by controlling both mirror geometry and growth, to control the spontaneous emitter""s radiative lifetime through mode confinement. We believe that this effect will ultimately provide significant improvements in lasing thresholds, speed, and efficiency over more conventional vertical cavity surface emitting laser (VCSEL) devices. By using high resolution microfabrication and carefully designed nanocavities, we have shown that we can define laser cavities in which over 85% of the spontaneously emitted light is coupled into a single lasing mode. This high spontaneous emission coupling factor (xcex2) is obtained at the expense of microfabricating mirrors by ion etching, resulting in structures with high surface to volume ratios.
Fabrication techniques for constructing two-dimensional photonic band gap crystals make it possible to control the propagation of light within a semiconductor material and to create optical microcavities with very small volumes.
Confinement of light in the vertical direction can be obtained by using a suspended membrane of high refractive index material surrounded by air on both sides. A two dimensional photonic crystal in the horizontal plane can then be defined into this wave-guiding slab to form an optical microcavity 36, and waveguides in the photonic crystals can be used to connect adjacent optical devices. Microcavities based on photonic crystals can therefore be used as light emitting devices with very small mode volumes, efficient microlaser sources, and in-plane microresonator networks.
Here we design and fabricate such in-plane membrane microresonators with the desired freedom in the geometrical design of the cavities as well as the possibility of efficient coupling. We consider the use of a triangular array of air holes in a slab of material with a high dielectric constant, which has been shown to exhibit a band gap for both transverse electric (TE) and transverse magnetic (TM) polarizations. The InGaAs/InGaAsP material system was chosen since it does not suffer from large surface recombination losses and is relatively easy to microfabricate structures into. It has previously been used to demonstrate optically and electrically pumped whispering gallery mode microdisk lasers with diameters down to 2 xcexcm and cavity thickness comparable to our designs.
The cavity structures are formed by combining localization in a thin high refractive index slab due to total internal reflection, and lateral confinement with the use of a two dimensional photonic crystal. The high index slab in our case is, designed to be half a wavelength thick in order to shrink the optical mode volume as much as possible. Two dimensional band structure calculations are first performed to establish approximate characteristics of a perfectly periodic two dimensional photonic crystal in an optically thin slab.
The invention and its various embodiments may now be better visualized by turning to the following drawings, wherein like elements are referenced by like numerals.